WBCHSE HS Biological Science flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Fluid mosaic model

Singer and Nicolson's 1972 model describing the plasma membrane as a fluid phospholipid bilayer with proteins and glycoproteins embedded in or attached to it, all free to move laterally.

Mitotic index

The proportion of cells in a sample that are in any stage of mitosis, expressed as a percentage of the total cells counted; used to compare cell division rates between tissues.

Crossing over

Exchange of corresponding segments between non-sister chromatids of homologous chromosomes during pachytene of prophase I of meiosis, producing new combinations of alleles on a chromosome.

Km (Michaelis constant)

The substrate concentration at which an enzyme works at half its maximum velocity. A low Km indicates high affinity of the enzyme for its substrate.

Competitive inhibition

An inhibitor resembling the substrate binds the active site, so more substrate is needed to reach a given rate. Apparent Km rises but Vmax is unchanged.

Peptide bond

A covalent bond formed between the carboxyl group of one amino acid and the amino group of the next, with loss of water; it links amino acids into a polypeptide chain.

Semi-conservative replication

Each parental DNA strand acts as a template for a new complementary strand, so every daughter double helix contains one old and one newly synthesised strand.

Test cross

A cross between an individual showing a dominant phenotype and a homozygous recessive individual, used to determine whether the dominant individual is homozygous or heterozygous.

Okazaki fragments

Short DNA pieces synthesised discontinuously on the lagging strand in the direction opposite to the replication fork, later joined by DNA ligase into a continuous strand.

Photolysis of water

Light-driven splitting of water at photosystem II in the thylakoid membrane, releasing electrons to replace those lost by P680, plus protons and oxygen gas.

Photorespiration

A wasteful process in which RuBisCO fixes oxygen instead of carbon dioxide, consuming ATP and releasing CO2; it is suppressed in C4 and CAM plants.

Pressure-flow hypothesis

The explanation of phloem transport in which sugars loaded at source raise solute potential, drawing water in and pushing sap by mass flow towards sink tissues.

Respiratory quotient

The ratio of carbon dioxide released to oxygen consumed during respiration; about 1.0 for carbohydrate, 0.7 for fat and 0.8 for protein.

Counter-current mechanism

Arrangement in the loop of Henle and vasa recta where fluid flows in opposite directions, maintaining a steep osmotic gradient in the renal medulla for urine concentration.

SA node

A patch of specialised cardiac muscle in the right atrial wall that spontaneously generates impulses, setting the heart rate and acting as the natural pacemaker.

Resting membrane potential

The potential difference of about minus seventy millivolts across a resting neuron membrane, maintained by the sodium-potassium pump and selective ion permeability.

Double fertilisation

The unique event in angiosperms where one male gamete fuses with the egg to form a zygote and another fuses with two polar nuclei to form triploid endosperm.

Corpus luteum

The glandular body formed from the ruptured ovarian follicle after ovulation; it secretes progesterone, which maintains the endometrium during early pregnancy.

Acrosome reaction

Release of hydrolytic enzymes from the acrosome of a sperm on contact with the zona pellucida of the ovum, allowing the sperm to penetrate and fuse with the egg membrane.

Hardy-Weinberg principle

In a large randomly mating population without mutation, migration or selection, allele and genotype frequencies remain constant from generation to generation; p plus q equals one.

Analogous organs

Structures with different evolutionary origins and basic plans but similar function and appearance, such as the wings of insects and birds; evidence of convergent evolution.

Directional selection

A form of natural selection in which individuals with an extreme value of a trait survive better, shifting the population mean in one direction over generations.

Carrying capacity

The maximum population size of a species that a given environment can support indefinitely, symbol K in the logistic growth equation, where growth rate becomes zero.

Competitive exclusion principle

Gause's principle stating that two species competing for exactly the same limiting resources cannot coexist indefinitely; one will outcompete and eliminate the other locally.

Ecological succession

The gradual, directional change in species composition of a community over time, from pioneer species to a stable climax community, driven by changing environmental conditions.

Biochemical oxygen demand

The amount of dissolved oxygen consumed by microorganisms decomposing organic matter in a water sample; a high value indicates heavy organic pollution.

Biodiversity hotspot

A region with exceptionally high species richness and endemism that has already lost a large proportion of its original habitat, making it a conservation priority.

Biological magnification

The progressive increase in concentration of a persistent, fat-soluble pollutant such as DDT at each successive trophic level of a food chain, harming top consumers.

Restriction endonuclease

A bacterial enzyme that recognises a specific palindromic DNA sequence and cuts it, producing sticky or blunt ends; a basic tool for recombinant DNA technology.

Secondary immune response

The rapid, stronger antibody response produced when a previously encountered antigen reappears, because memory B and T cells are already present from the first exposure.

Bt cotton

A transgenic cotton variety carrying a gene from Bacillus thuringiensis that makes the plant produce a toxin lethal to bollworm larvae, reducing insecticide use.

Rf value

In chromatography, the distance travelled by a solute divided by the distance travelled by the solvent front from the same origin; a characteristic identifier under fixed conditions.